'Solar fuel' research mimics photosynthesis

NEW HAVEN, Conn.--In an ambitious attempt to replicate nature, various researchers are seeking to create fuels from water and sunlight, much the way plants do.

California Institute of Technology professor Nate Lewis on Saturday gave a snapshot of the "swing for the fences" research his lab is pursuing to make fuels directly from water and sunlight. Caltech last year was picked as the lead for a newly created Joint Center for Artificial Photosynthesis (JCAP) to run the Department of Energy's Fuels from Sunlight Energy Innovation Hub.

Researchers at the Joint Center for Artificial Photosythesis are designing arrays of microwires coated with catalysts that can split water to make hydrogen or liquid hydrocarbon fuels.
CalTech

During a talk at the Yale Climate & Energy Institute's annual conference, Lewis described the concepts driving his research and what form a solar fuel generator could take.

The sun is the largest source of energy, but storing solar energy with conventional means, such as batteries, is very expensive, he said. The notion behind his research is to store solar energy in the chemical bonds of fuels. Light-duty transportation will move toward electric vehicles because they are more efficient than internal combustion engines, but there is still a need for liquid fuels in other forms of transportation or to generate power when there is no sun.

"It's inevitable that we will find a way to efficiently take the biggest energy source we have in the sun and store it in chemical fuels, thereby obviating the storage problem, thereby having a drop-in replacement fuel, and thereby solving the (fuel) infrastructure problem," he said. "We are going to do this. The question is how fast and how soon."

The center's solar fuel generator is designed around tiny wires of silicon placed in a solution. Traditional solar cells also use silicon, which releases a flow of electrons, or electricity, when hit by light. But the work at Lewis' lab diverges from the solar cell form and function in significant ways.

Using nanotechnology, the design calls for rods of silicon "microwires," which allows the material to absorb more light. Rather than only create electricity, these fibers are treated with a catalyst to use the sun's energy for fuel production. A catalyst can react with water to produce hydrogen gas, which can be used as a fuel. A longer-term goal is to discover catalysts that use carbon dioxide from the air in the production of a liquid hydrocarbon fuel such as methanol.

The design intentionally copies the process of photosynthesis, where leaves split water and use carbon dioxide from the air to make sugars. But researchers like Lewis are seeking to greatly increase the efficiency of converting sunlight to stored energy. Plants spend much of the sun's energy on growing.

Lewis said that arrays of these microfibers could be placed on a flexible polymer so that they would resemble a "solar carpet" where the fibers are like tiny trees in a forest. How an entire fuel-producing system would be designed and what fuel is more economical to produce is still not clear, he said.

"We don't actually know whether or not it's cheaper to wick out gas at low pressure or to wick out liquid (fuel). It depends on how we plumb it and the only way to find out is to build a few and cost them," he said.

Research in artificial photosynthesis has been going on for decades. One of the biggest technical challenges is finding materials that are robust enough to sit in the sun for several years, yet are cheap and efficient at converting sunlight to electrical energy, he said. The goal is to be robust, cheap, and efficient, but scientists now can get only two of the three.

The Fuels from Sunlight research is a long-term effort, but Lewis said a prototype solar carpet could be ready in two years and he is confident that the efficiency can ultimately be as good at many of today's solar panels in converting sunlight to usable energy.

Artificial leaf
Outside the Fuels from Sunlight lab are many other research and commercial programs, which could also be called solar fuels.

Notably, there are a number of biotech-oriented efforts to use genetically manipulated microorganisms such as bacteria to make fuels directly from sunlight and water. Joule Unlimited, for example, is a start-up that is designing a bioreactor in the shape of a flat plastic pane that grows cyanobacteria from water, nutrients, carbon dioxide, and sunlight. The cyanobacteria are genetically engineered to make diesel fuel directly.

A picture of the tiny silicon wires that would be put in water to create hydrogen from sunlight.
Caltech

Another high-profile researcher in artificial photosynthesis is Daniel Nocera from the Massachusetts Institute of Technology. He co-founded a company called Sun Catalytix that is using a relatively inexpensive catalyst to split water to make hydrogen and oxygen. The company plans to make a system that can make the hydrogen, store it, and then run it through a fuel cell to make electricity or burn it for heat and power. It envisions the technology would first be suited for distributed energy systems in developing countries.

Two weeks ago at the National Meeting of the American Chemical Society, Nocera presented results from an effort to make an "artificial leaf." Sun Catalytix has already made a prototype system where a photovoltaic solar cell, coated with a catalyst and placed in water, can make hydrogen and oxygen.